There are, currently available, motors having circular arrays of permanent magnets arranged on a rotor. The rotor is associated with a stator having a plurality of pole pieces energized by a coil carrying a controlled electric current. Commutation of the electric current produces a rotating magnetic field in the stator, and interaction between the pole pieces and the permanent magnets results in rotation of the rotor. One example of such a motor is described in U.S. Pat. Ser. No. 4,237,396, issued to Blenkinsop et al. on Dec. 2, 1980.
In designing motors of this general type, it is important to maintain the mutual inductance of the coil at the lowest possible level in order to obtain maximum utilization of the available magnetic flux, increasing the efficiency of operation and decreasing energy losses. Location and design of the pole pieces is a critical factor in maximizing the utilization of the magnetic flux. It is important that as many pole pieces as possible be arranged around the stator coil. Coincidentally, interpole magnetic flux leakage needs to be kept as low as possible to reduce leakage inductance.
Blenkinsop proposed staggering the arrangement of the pole pieces so that those pole pieces which open radially inwardly are radially further from the central axis of the machine than those pole pieces which open radially outwardly. Such arrangement increases the number of pole pieces which can be incorporated in a stator of given radius and prevents any increase in interpolar flux leakage resulting from the increased number of pole pieces. However, it will be appreciated that the dimensions of the machine are inevitably increased as a result of such staggering and this may be unacceptable in certain circumstances. An alternative proposal put forward in the same patent specification is the shortening of the radially inwardly opening pole pieces, again allowing an increase in the number of pole pieces, without increasing interpolar flux leakage. However, as a result of such shortening, the face area of the pole pieces is reduced, resulting in a reduction in the utilization of magnetic flux.
The present invention is directed to overcoming one or more of the problems as set forth above.